1. Field of the Invention
The present invention relates to a wavelength converter capable of obtaining a stable high output visible light laser beam by combining a fiber laser and a wavelength conversion element, and a two-dimensional (2D) image display device using this wavelength converter as a light source.
2. Description of the Background Art
A visible light source capable of emitting a highly monochromatic W-class high output is being required to realize large-size displays, high-luminance displays, etc. High-output red semiconductor lasers used in DVD recorders and the like can be utilized as small-size light sources having high productivity for red light out of three primary colors of red, green and blue. For green and blue light sources, however, realization by semiconductor lasers and the like is difficult and small-size light sources having high productivity are still asked for. Above all, it is highly difficult to realize green light sources since there is no material suitably usable for semiconductor lasers to obtain green output beams.
Wavelength converters as combinations of fiber lasers and wavelength conversion elements are realized as low-output visible light sources. The visible light source includes a semiconductor laser as a light source for excitation light for exciting the fiber laser and a nonlinear optical crystal as the wavelength conversion element, and is well-known as a small-size light source for green or blue light.
However, several problems need to be solved in order to obtain W-class high-output green and blue beams using such a wavelength converter. For example, in the case of obtaining a green output beam using the construction of a conventional wavelength converter, the wavelength converter needs to include a fiber laser for outputting a fundamental wave, a wavelength conversion element for converting the fundamental wave into a green laser beam and a lens for condensing an output of the fundamental wave to an end surface of the wavelength conversion element.
A basic laser operation of this fiber laser is further described. First, an excitation light from an excitation laser light source is incident on one end of a fiber. The excitation light incident on the fiber is absorbed by a laser-active material contained in the fiber and, thereafter, a seed light of the fundamental wave is generated in the fiber. This seed light of the fundamental wave reciprocates by being reflected many times in a resonator using a fiber grating formed in the fiber and a fiber grating of another fiber as a pair of reflection mirrors. Simultaneously, the seed light is amplified by a gain by the laser-active material contained in the fiber to increase its light intensity and to have a wavelength selected, thereby reaching a laser oscillation. It should be noted that the two fibers are connected by a connecting portion and the laser light source is current-driven by a laser current source for excitation.
Next, a basic operation of the wavelength converter is described. The fundamental wave is outputted by the fiber laser as described above to be incident on the wavelength conversion element via the lens. This fundamental wave from the fiber laser is converted into a harmonic by the nonlinear optical effect of the wavelength conversion element. The converted harmonic is partly reflected by a beam splitter, but the other part having passed through the beam splitter becomes a green laser beam as an output beam of the wavelength converter.
The harmonic partly reflected by the beam splitter is utilized by being converted into an electrical signal after being received by a light receiving element for the monitoring of the output beam from the wavelength converter. An output controller regulates a drive current of the laser light source by means of a laser current source for excitation so that the intensity of the converted signal becomes an intensity to give a desired output in the wavelength converter. Then, the intensity of the excitation light from the laser light source is regulated and the output intensity of the fundamental wave from the fiber laser is regulated, with the result that the output intensity of the wavelength converter is regulated. In this way, a so-called automatic power control (hereinafter, abbreviated as “APC”), in which the output intensity of the wavelength converter is kept constant, is stably performed.
In order to obtain a W-class high-output green laser beam, i.e. in order to increase the light output of the wavelength converter by such a construction, the fundamental wave of the fiber laser and the output of the excitation light need to be increased. On the other hand, since the fiber laser is formed to have a narrow and long shape by a laser medium and has a large surface area, it is originally formed to easily radiate heat generated thereby, but it is known that the efficiency of the fiber laser decreases by the heat generation of the rare-earth doped fiber if the fundamental wave and the output of the excitation light are increased. In order to prevent such an efficiency reduction of the fiber laser, there have been conventionally proposed a method for improving heat radiation by winding a fiber around a metallic reel (see, for example, Japanese Unexamined Patent Publication No. 2004-356421) and the like. On the other hand, there has been also proposed a method for retaining a rare-earth fiber in an adhesive film (see, for example, the specification of Japanese Patent No. 2888157) in order to facilitate the handling of the rare-earth doped fiber in the fiber laser whose output is in the level of several hundreds mW. A construction for not causing excitation at a fiber end face (see, for example, Japanese Unexamined Patent Publication No. 2001-156363) has been also proposed.
However, in a fiber laser light source in which the output of a fundamental wave exceeds 7 W, it has been difficult to efficiently radiate heat generated by a fiber and to improve a conversion efficiency from an excitation light to an emission light. In the case of using a metallic reel as in the prior art, there have been problems of higher parts cost and production cost and difficult miniaturization. On the other hand, it may be thought to arrange the fibers in such a manner as not to overlap each other for better heat radiation. However, this makes the handling more difficult and requires a larger space, thereby presenting a problem of difficult miniaturization.